Removal of acid gas, such as carbon dioxide (CO2) and/or hydrogen sulphide (H2S) from natural gas, commonly termed “gas sweetening” is a well known technology. There are several commercial technologies available for this purpose such as absorbent mediums or solvents (i.e. amines, glycol), physical solvents, membranes, cold processes, etc. Such an absorbent medium can also be referred to as a lean absorbent, prior to the absorption of acid gas from the natural gas, or a rich absorbent, after the absorption of acid gas from the natural gas.
Chemical solvents such as amines are widely used and extensive operating experience has been gained. Examples of applicable absorbents comprise amine based absorbents such as primary, secondary and tertiary amines; one well known example of applicable amines is mono ethanol amine (MEA). The liquid diluent is selected among diluents that have a suitable boiling point, are stable and inert towards the absorbent in the suitable temperature and pressure interval. An example of an applicable diluent is water. Examples of suitable amines for use with a diluents such as water are aqueous solutions of monoethanolamine (MEA), methylaminopropylamine (MAPA), piperazine, diethanolamine (DEA), triethanolamine (TEA), diethylethanolamine (DEEA), diisopropylamine (DIPA), aminoethoxyethanol (AEE), dimethylaminopropanol (DIMAP) and methyldiethanolamine (MDEA), methyldiisopropanolamine (MDIPA), 2-amino-1-butanol (2-AB) or mixtures thereof. The removal process comprises a closed circulation loop containing the solvent. In an amine based absorption process the CO2/H2S reacts with the amine in an absorber unit and is bound strongly to the solvent. The reaction between the amine and the acid gas is strongly exothermic. The solvent can be regenerated, often by combining flash regeneration by pressure reduction and thermal regeneration by supplying heat in a stripper, where the CO2/H2S is released from the solvent. The regenerated solvent is passed through suitable compressor and heat exchange units for pressurization and temperature adjustment, and is returned to the absorber unit. A typical, conventional amine plant using an absorber column is schematically shown in FIG. 9.
In the prior art arrangement shown in FIG. 9, there is provided an acid gas removal process wherein a sour gas stream containing undesirable hydrogen sulphide (H2S) and carbon dioxide (CO2) is introduced to a contact absorber A through line 901. As the sour gas flows upward through absorber A the sour gas contacts downward flowing mixture of normal lean amine which is introduced to the absorber through line 902. The process gas has most of the acid gases removed by the time it leaves the absorber after contacting the lean amine from line 902. A product gas (sweet gas) having a substantially reduced content of the hydrogen sulphide and carbon dioxide is withdrawn from the top of the absorber via line 903.
A stream of rich amine solution containing absorbed hydrogen sulphide and carbon dioxide as salts of amine is removed from the absorber through line 904. The pressure of the solution is reduced and it then goes to a rich amine flash tank C. The flash gases exit through line 905 and the rich amine solution exits through line 906. The rich amine stream passes through a lean/rich heat exchanger D to recover sensible heat from hot lean amine and is then introduced to an amine regenerator stripping column B through line 907. Internal stripping steam is generated by reboiling the amine solution in a stripper reboiler, or a heat exchanger E, using a suitable heat medium 908. The lean amine temperature can vary from about 100° C. to 140° C., depending on the type of amine, its concentration and its pressure. The steam generated from the reboiled amine is introduced near the bottom of stripping column B through line 911 and passes upward through the amine solution providing heat to decompose the hydrogen sulphide and carbon dioxide amine salts and the stripping vapour to sweep the acid gas away from the amine solution and out of the stripping column. The mixture of steam, hydrogen sulphide, and carbon dioxide exits the stripper overhead through line 910.
A hot lean amine stream 909 exits the bottom of the stripper B, is passed through the lean/rich heat exchanger D and through a cooler F where the lean amine solution is cooled to a temperature of about 35° C. to 55° C. The cooled lean amine stream in line 912 continues through line 902 to the top of the absorber A.
Current process equipment as used at e.g. at the Sleipner T installation is very large. In this case, the absorber column inner diameter is almost 4 m and the total height is almost 18 m. The footprint and weight of the absorber column are thus significant. Depending on the applied amine and amount of acid gas to be removed the circulation rates on the solvent will also be significant. This requires a significant amount of power for pumping, heating and cooling of the circulating amine solution. Amine solutions are also known to be corrosive, especially in the CO2 rich parts of the process. Depending on operating conditions and impurities accumulated in the solution, the amine is susceptible to degradation and contamination. Equipment using amines may also experience various types of failure caused, for instance, by foaming or by insufficient contact between gas and liquid. Foaming of the gas and liquid, caused by e.g. condensation of hydrocarbon or solids suspended in the gas after insufficient pre-filtration, is also a known issue in conventional absorbers. Carry-over of amine droplets in the sweet gas from the absorber to downstream equipment is another cause of foaming. A further problem with the absorber column relates to the importance of maintaining good contact between the natural gas and the liquid amine and to provide a good liquid distribution in order to achieve an effective removal of acid gas.
The object of the invention is to solve the above problems by providing an improved process for the removal of acid gas from natural gas by means of a absorbent medium, and for the regeneration of the absorbent medium by the removal of acid gas from the absorbent.